The present invention relates to the wrapping of packages with sheet material. More specifically, the invention relates to a method and apparatus for wrapping with sheet material a package lacking substantial rigidity, e.g., one or a bundle of textile or paper products. The invention finds particularly advantageous application to the wrapping of packages lacking substantial rigidity and which are readily compressible, e.g., a package of stacked of napkins.
A well known method of packaging napkins and similar products for retail sale is to wrap the product in protective sheet material such as polyethylene. With respect to napkins, typically such a packaging operation takes place as the final step in an automated assembly line operation involving initial napkin forming and stacking operations.
A desirable characteristic of most wrapped consumer packages, including packages of napkins, is that the product be tightly wrapped. A tight package wrapping provides a neat appearance with added consumer appeal, and assists in maintaining package integrity. A problem which frequently arises with wrapped packages of bulked product, e.g. napkins, is that the bulked product experiences a degree of settling between the time it is packaged and the time it is offered for sale to the consumer. Hence, a package which was initially tightly wrapped may become loose and sloppy in appearance by the time it reaches the retail sales shelf. To combat the tendency of napkin packages to become loose due to settling, it is desirable to compress the napkin package (one or more stacks of napkins) to a significant degree before it is wrapped. In this manner, the inherent elasticity of the bulked package presses the package tightly against the wrapping and further reduction of the height of the package is minimized.
Imparting compression to the product during the wrapping operation also advantageously produces a smaller finished package requiring less wrapping material and having reduced shipping costs.
A piece of equipment widely used for automated packaging of the type described above is manufactured and sold by Hayssen Manufacturing Co., of North Sheboygan, Wis., as the Model 5200 overwrap machine. The structure and sequential operation of this device is schematically illustrated in prior art FIGS. 13A-13G, and described in summary fashion below.
Referring to FIG. 13A, a package in-feed conveyer (not shown) has just delivered a "twin pack" of napkins 3 (two side-by-side stacks of napkins--only one is visible) to elevator mechanism 5. An elevator plate 7 of mechanism 5 reciprocates up and down, on an elevator shaft 8, in a synchronized manner under the control of a rotating cam 10. In the position shown, elevator plate 7 is positioned at the bottom of its stroke so that a napkin package 3 may be received thereabove on support rails 12 extending into elevator well 14. At this time, a piece of film 9 of the wrapping material, e.g., polyethylene, has been automatically cut from a parent roll (not shown) and positioned across elevator well 14 over the top of napkin package 3 by a belt conveyor (not shown).
Referring now to FIG. 13B, napkin package 3 is lifted by elevator mechanism 5 into contact with film 9 such that film 9 is wrapped around the top of the napkin package to drape over the front and rear (longitudinal) sides of the napkin package, and to protrude outwardly from the lateral sides. As seen in FIG. 13C, elevator plate 7 continues its upward movement pressing napkin package 3 against a spring loaded pressure plate 11 positioned at the desired package height. When elevator plate 7 reaches the top of its stroke as shown in FIG. 13C, napkin package 3 has been raised to a position above front and rear underfolders (bottom tucking blades) 13a,b. This allows the underfolders to move by cam action under the front and rear edges of napkin package 3 in order to fold or tuck front and rear film edge portions 9a,b underneath napkin package 3. Underfolders 13a,b also provide support for the napkin package when elevator plate 7 is lowered. At this point, package 3 has been compressed a certain amount.
FIG. 13C illustrates the tendency of the lower edges 3a of napkin package 3 to fold or cuff around elevator plate 7 during the compression. This occurs because elevator plate 7 is significantly narrower than the stacked napkins so that, as shown in FIG. 13D, elevator plate 7 may pass between underfolders 13a,b as elevator mechanism 5 is lowered. As a result, the degree to which pack 3 may be compressed during the packaging operation is significantly limited. If substantial compression is attempted, the resultant cuffed edges will be hit by tucking blades 13 as they move under the package. This results in defective finished packages having rolled over napkin edges inside.
After elevator plate 7 has been lowered below tucking blades 13, napkin package 3 remains supported at its edges by underfolder 13. Then, as shown in FIG. 13E, rear underfolder 13b moves further inward toward front underfolder 13a, tucking film 9 under the bottom of napkin package 3 and pressing rear film edge portion 9b flat against the bottom of the napkin package. Next, as shown in FIG. 13F, a transfer device 15 carrying spring loaded pressure plate 11 slides napkin package 3, with film 9 wrapped therearound, laterally to a bar-type heat sealer 17. This sliding movement brings front film edge 9a flat against the bottom of napkin package 3 in overlapping relation with edge portion 9b, and positions the overlapping portions 9a,b directly above heat sealer 17 for heat sealing therealong.
Omitted for clarity in FIGS. 13A-G are vertically oriented side folders (side tucking blades) which advance by cam action along the lateral sides of the napkin stacks slightly after rear underfolders 9b have begun to move for the second time. These side folders serve to fold or tuck in the rearward portions of firm 9 protruding from the lateral sides of the package. This constitutes the first step in a progressive side folding process described further below.
The machine cycle is completed by the return of transfer device 15, elevator plate 7 and underfolders 13 to their original positions as shown in FIG. 13G. In this position, elevator mechanism 5 is set to receive another napkin package from the package in-feed conveyor and a new piece of film 9 is advancing to a position over the elevator well. Although not illustrated, during the next cycle, the package just heat sealed in the previous cycle will be pushed by the transfer mechanism between a pair of side folding plates to complete the side folding process. These plates first tuck inwardly the frontward portion of film 9 protruding from the lateral sides of the napkin package, and then complete standard envelope folds on the lateral sides of the package. Thereafter, the package is advanced between a pair of heat sealing belts (not shown) where the envelope folds are heat sealed to produce a finished package.
Further details of the above-described prior art process and machine will be evident from the detailed description of the invention herein, wherein commonalities of the present invention and the prior art process and machine are identified.
The above-described prior art process and machine are capable of producing "twin pack" packages of napkins which are very popular with consumers. With this type of package configuration, the wrap is draped over the top face of two side-by-side napkin stacks (e.g., seventy napkins each) and a linear heat seal is provided across the bottom face of the stacks. Envelope folds are provided along two opposite napkin edge surfaces (the lateral package sides). While arrangements have been developed for providing high compression to stacked packages wrapped in a direction orthogonal to the stacking direction (see, e.g., U.S. Pat. No. 3,006,119 to Fingerhut), heretofore a satisfactory method and apparatus for wrapping stacked packages in the stacking direction (as in the above-mentioned "twin packs"), with an optimum amount of compression, has not existed.
The following U.S. patents disclose other packaging methods and apparatus operative to wrap a stacked package in the stacking direction.
James et al. U.S. Pat. No. 4,279,116 and Coates U.S. Pat. No. 3,212,230 each disclose wrapping apparatus which utilize complex linkages and cam-actuation to expand and contract an elevator plate so that the entire package to be wrapped is supported during the wrapping operation. In each device, the elevator plate can be contracted to allow underfolders to engage and fold wrapping material under the package. While these patents address the problem of wrapping defects resulting from curled or folded edges of the product, they do not address the need to provide significant compression to a soft bulked product during the wrapping operation to achieve a tight package.
It is also significant that the complex cam-actuated expandable elevator mechanisms of these patents would not be easily adaptable to retrofit existing apparatus, such as the Hayssen Model 5200 wrapper.
Gordon U.S. Pat. No. 3,861,120 discloses a wrapping apparatus for wrapping a stack of napkins, having an air cylinder actuated upper gripping member serving to compress the napkin stack as it is raised by an elevator. The gripping member also serves to move the package first in one horizontal direction and then in a reverse direction to effect underfolding of the wrapper about the package, without the requirement of retractable underfolders. Without retractable underfolders, this apparatus is able to employ an elevator plate corresponding in size to the napkin stack. While this arrangement addresses the problem of wrapping defects due to sagging package edges, the need to move the package in two horizontal directions across the elevator plate to form the underfolds has apparent disadvantages. For example, the reversal of motion of the package would create a greater likelihood that the wrap will become displaced on the package, resulting in defective folds. Secondly, the machine operation would be made more violent by the added motion reversal, leading to increased machine break-down and required maintenance. Also, each additional machine stroke required for the packaging operation increases cycle times and lowers production efficiency.
It is also significant that the Gordon apparatus is complex and would not be easily adaptable to existing apparatus such as the Hayssen Model 5200 wrapper.